1,4-Butanediol is a glycol that can be used to produce polybutylene succinate by polycondensation with succinic acid or a derivative thereof. Further, 1,4-butanediol can be used to produce polybutylene terephthalate by polycondensation with terephthalic acid or a derivative thereof; and also to produce poly(butylene adipate/terephthalate) by polycondensation with adipic acid or a derivative thereof and terephthalic acid or a derivative thereof.
A known method for synthesizing polybutylene succinate includes the steps of: causing an esterification reaction of succinic acid or a derivative thereof with 1,4-butanediol; and subsequently heating and stirring the resulting ester in vacuo to carry out a transesterification (i.e., polycondensation) reaction. During the esterification step, a carboxyl group of succinic acid or a terminal group of a derivative thereof is subjected to an esterification reaction with a hydroxyl group of 1,4-butanediol at a predetermined temperature and pressure so as to produce an oligomer having a terminal hydroxyl group.
In the polycondensation step, transesterification proceeds under the presence of a catalyst, and 1,4-butanediol contained as a byproduct is devolatilized under a reduced pressure. This process allows the oligomer to be converted into a polymer with high viscosity. Because of this, it is necessary for the above process to keep the reaction system at a high temperature and satisfy a condition of the reduced pressure so that a degree of vacuum is more increased as the reaction proceeds to latter stages. At this time, in order to distill away a desorption component present on the surface of the reactant, mechanical stirring should be performed, so as to sufficiently increase an evaporating surface area of the byproduct and a rate of migration (i.e., a surface renewal rate) of the byproduct from the inside of the reaction liquid to the evaporating surface.
Likewise, the polybutylene terephthalate and the poly(butylene adipate/terephthalate) can be synthesized by esterification, followed by polycondensation.
In the polycondensation step, a molecular chain of the polymer is elongated via transesterification caused by heating and stirring of the ester under reduced pressure. At this time, 1,4-butanediol contained as a byproduct is devolatilized and removed under reduced pressure. This facilitates the reaction to proceed. However, the above process has a drawback of preventing the transesterification from proceeding as described below. That is, a low-molecular-weight oligomer scatters accompanied by 1-4-butanediol. The scattering oligomer attaches to a pipe of an exhaust system and an inner wall of a condenser to occlude the exhaust system.
Consequently, this occlusion deteriorates a degree of vacuum in a polycondensation reactor and prevents the progress of the transesterification.
As a solution to the above drawbacks, widely known is a method (Patent Literature 1) including: providing an ejector driven by 1,4-butanediol vapor and a condenser arranged at a downstream of the ejector of an exhaust system in a polycondensation step; and further providing a hot well tank connected to the condenser.
The method according to Patent Literature 1 makes it possible to wash away scattered substances in the exhaust system generated in the reaction process. However, 1,4-butanediol, which has a melting point of 20° C., is readily solidified under, for example, reduced pressure at room temperature. Therefore, if a suitable temperature condition is not maintained, 1,4-butanediol injected via the ejector turns to solidified, so that this may instead promote occlusion of the exhaust system. In addition, the technology according to Patent Literature 1 primarily aims at separating a solvent (e.g., THF) included in the exhaust system. Accordingly, although the scattered substances generated in process are collected in a hot well tank, those substances are not to be recycled. Hence, a product yield based on the starting material cannot be improved.